1. Field of the Invention
The present invention relates to a fixing apparatus in an image forming apparatus such as a printer or a copier which employs electrophotography.
2. Description of the Related Art
Some known fixing apparatuses provided in an image forming apparatus such as a copier or a laser beam printer, is an apparatus use a film. Generally, such a fixing apparatus includes the film in a tubular shape, a heater of a plate shape which is in contact with an inner surface of the film, and a pressing member that forms a nip portion together with the heater across the film. The fixing apparatus executes fixing processing in which a recording medium with a toner image formed thereon is heated while being conveyed through the nip portion, so that the toner image is fixed on the recording medium. The fixing apparatus uses a film with a low heat capacity and thus can be warmed up in a short period of time, so that a first print out time (FPOT) of the image forming apparatus can be shortened.
It has been widely known that when the film with a low heat capacity is used, temperature in a sheet non-passing portion rises, that is, an excessive temperature rise in a portion where a recording medium does not pass through is likely to occur in the fixing apparatus. In view of the above, Japanese Patent Application Laid-Open No. 11-84919 discusses a configuration for preventing the temperature rise in the sheet non-passing portion. More specifically, the temperature rise in the sheet non-passing portion is prevented by bringing a heat conducting member such as an aluminum alloy into contact with a surface of the heater disposed opposite to a surface of the heater contacting the film, to increase thermal conductivity in a longitudinal direction.
A fixing apparatus of the type described above includes a heater holder having a trench portion in which the heater is accommodated. The trench portion has a larger width than the heater in consideration of a component tolerance. Thus, while the film is rotating, the heater moves to contact a surface of the trench portion on a downstream side in a conveyance direction of a recording medium. As a result, a gap is formed between a surface of the trench portion on an upstream side and an end surface of the heater. When this gap is wide, foreign substance such as a staple attached to the recording medium, might enter the gap and form a hole in the film (Japanese Patent Application Laid-Open No. 2012-123329). In view of the above, in some apparatuses, two protrusions serving as positioning portions for a heater in a direction orthogonal to the conveyance direction of the recording medium, and a protrusion that is disposed between the two protrusions and prevents warpage of the heater are provided on the surface of the trench portion facing the end surface of the heater on the downstream side in the conveyance direction of the recording medium. Such protrusion of a small width can be accurately manufactured and its gap can be made smaller easily. Furthermore, the protrusion for preventing the warpage of the heater prevents the warpage of the heater, and thus prevents the gap from widening. Thus, in such a known fixing apparatus, the gap can be kept small in a stable manner, so that the foreign substances can be prevented from entering the gap.
However, the fixing apparatus in which the heat conducting member and the heater are accommodated in the trench portion of the heater holder has the following problem. When frictional force produced by the rotation of the film is applied to the heat conducting member through the heater, the heat conducting member warps in the conveyance direction of the recording medium and is brought into contact with the protrusion for preventing the warpage of the heater. When the heat conducting member in this state thermally expands, the heat conducting member may deform to lift from the heater. When this happens, the effect in preventing the temperature rise of the sheet non-passing portion is reduced. In other words, the fixing apparatus including the holding member in which the heat conducting member and the heater are accommodated in the trench portion has a problem in that it is difficult to achieve both prevention of entrance of foreign substances into the gap between the trench portion and the heater and the maintain the effect in preventing the temperature rise of the sheet non-passing portion.